Adult male Marvelous Spatuletails (Loddigesia mirabilis) have greatly specialized tail feathers (rackets) that are used in dramatic displays. We compared adult males' tail structure with that of immature males and females, and with that of related species, to determine the nature of adaptations in the tail. We provide the first detailed description and inference of the function of the rectricial bulb of the tail in hummingbirds. Metamorphic enlargement of the outer rectrices (pair 5) of male Marvelous Spatuletails first occurs before the body plumage is fully adult. The first two rackets apparently are grown sequentially; in subsequent, definitive molts they may be grown simultaneously. Vanes of the other postjuvenal rectrices (pairs 1–4) are small or rudimentary, and pairs 2–4 serve no aerodynamic function. However, the calami of these feathers contribute to the general structural complex of the rectricial bulb. Intrinsic muscles of the bulb govern spreading of the rackets, whereas extrinsic muscles mainly govern elevation, depression, and tilting of the entire bulb and its imbedded rectrices. Together, these muscles effect the spectacular movements of the racket head. The shape and structure of the rackets contribute to unique features of the display. The tails of females and immature males are well suited to maximize lift:drag ratios during normal flight, whereas the elaborate tail of the adult male probably reduces this ratio. An elongate pair of central, ventral major tail coverts may replace some of the aerodynamic function lost in the rudimentary rectrices of the adult male.